EID and RFID systems which are the subject of the present invention include a signal emitter or reader which is capable of emitting a high frequency signal in the kilohertz (kHz) band range or an ultra-high frequency signal in the megahertz (MHz) band range. The emitted signal from the reader is received by a transponder which responds to or is activated in some manner upon detection or receipt of the signal from the reader.
Generally, the transponder for an EID system will include signal processing circuitry mounted on a printed circuit board which is attached to an antenna, such as a coil. The signal processing circuitry can include a number of different operational components as known in the art, though many operational components can be included in a single integrated circuit.
Certain types of "active" transponders may include a power source such as a battery which may also be attached to the circuit board. The battery is used to power the signal processing circuit during operation of the transponder. Other types of transponders such as "Half Duplex" ("HDX") transponders include an element for receiving energy from the reader such as a coil, and an element for storing energy, for example a capacitor. In an HDX system, when the signal from the reader is, turned off the capacitor discharges into the circuitry of the transponder to power the transponder so it can emit or generate a signal. The circuit designs for both active and HDX transponders are known in the art and therefore they are not described in detail herein.
Finally, a "Full Duplex" ("FDX") transponder generally does not include either a battery or an element for storing energy, instead energy is induced into the antenna or coil and used to power the signal processing circuitry of the transponder and generate the response concurrently with the emission of the emitted signal from the reader. FDX transponder circuits are also known in the art, an early example being disclosed in U.S. Pat. No. 4,333,072.
For each of the types of transponders presently in use, there is often a need for an application whereby the entire transponder must be encased in a sealed member so as to allow implantation into biological items to be identified, or for use in submerged or corrosive environments. Accordingly, various references, including U.S. Pat. Nos. 4,262,632; 5,025,550; 5,211,129; 5,223,851 and 5,281,855, disclose completely encapsulating the circuitry of various transponders within a ceramic, glass or plastic closed-ended cylinder.
For an encapsulated transponder, it is generally the practice to assemble the transponder circuitry and then insert the circuitry into a glass or plastic cylinder, one end of which is already sealed. The open end of the cylinder is then melted closed using a flame, to create an hermetically sealed encapsulant. However, the flame sealing of the encapsulant is a labor intensive, and therefore relatively expensive operational step in the construction of the transponder.
To prevent the circuitry from moving around inside of the glass encapsulant, it is also known to use a small amount of epoxy to bond the circuitry of the transponder to the interior surface of the glass encapsulant. A problem arises, however, in using epoxy to bond the circuitry of the transponder to a glass encapsulant. Due to the differences in manufacturing tolerances, it is difficult to determine exactly how much epoxy is needed in various applications. The displacement volume of the circuitry varies depending on the tolerance associated with the dimensions of the signal processing circuitry, circuit board and antenna or coil. In addition, the inside diameter of the glass encapsulant also changes due to variations in its tolerances.
In order to complete the sealing of the glass encapsulant, it is necessary to use an amount of epoxy which will leave a gas or air space above the top of the epoxy in order to assure complete hermetic sealing. As a result, the inside of the glass encapsulant is only partially filled with an epoxy and the circuitry of the transponder extends above the parting line between the top of the epoxy material and the entrapped gas or air. This creates a cantilever effect whereby the mass of the extending portion of the circuitry can oscillate, potentially disrupting the continuity of the electrical circuitry and breaking the delicate conductors on the circuit board or of the antenna or coil itself.
Accordingly, it would be beneficial to have a new encapsulant design for an EID or RFID transponder which eliminates costs and problems associated with flame sealing of an all glass encapsulant, and yet allows the transponder to be used in the implanted, submerged or corrosive environments for which it is intended.